Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
  • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
  • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
  • H04L1/0618—Space-time coding

Definitions

• This invention relates to differential space- time block coding, for example for a wireless communications system.
• wireless communications channels are subject to time-varying multipath fading, and it is relatively difficult to increase the quality, or decrease the effective error rate, of a multipath fading channel. While various techniques are known for mitigating the effects of multipath fading, several of these (e.g. increasing transmitter power or bandwidth) tend to be inconsistent with other requirements of a wireless communications system.
• One technique which has been found to be advantageous is antenna diversity, using two or more antennas (or signal polarizations) at a transmitter and/or at a receiver of the system.
• each base station typically serves many remote (fixed or mobile) units and its characteristics (e.g. size and location) are more conducive to antenna diversity, so that it is desirable to implement antenna diversity at least at a base station, with or without antenna diversity at remote units. At least for communications from the base station in this case, this results in transmit diversity, i.e. a signal is transmitted from two or more transmit antennas .
• a disadvantage of the STBC technique as described by Alamouti is that it requires estimation of the communications channel. While this can be done for example using pilot signal insertion and extraction, this is not desirable, for example because the pilot signal requires a significant proportion of the total transmitted power of the system.
• this invention provides a method of differential space-time block coding comprising the steps of: producing, from symbols to be encoded, successive space-time blocks H x (Xi) each of T symbols in successive symbol intervals on each of T paths in accordance with a T by T orthogonal matrix H x , where T is an integer greater than one, Xi represents the symbols to be encoded in a space-time block, and i is an integer identifying each space-time block; producing differentially encoded space-time output blocks H z ,i each of T symbols in successive symbol intervals on each of T output paths; and delaying the differentially encoded space-time output blocks H Z(i to produce respective delayed blocks H Z/i - ⁇ ; each differentially encoded space-time output block H z ,i being produced by matrix multiplication of the block H x (Xi) by the delayed block H z ,i- ⁇ .
• T 2 and two symbols are encoded in each space-time block.
• T 4 and three symbols are encoded in each space-time block.
• the step of producing the successive space-time blocks H x (Xj . ) comprises a multiplication of the symbols to be encoded by a normalization factor.
• the symbols to be encoded comprise M-ary phase shift keying symbols, where M is an integer greater than one.
• a differential space-time block coder comprising: a space- time block coder responsive to symbols to be encoded to produce successive space-time coded blocks; a matrix multiplier having a first input for said successive space- time coded blocks, a second input, and an output providing differentially encoded space-time blocks; and a delay unit for supplying each differentially encoded space-time block from the output of the matrix multiplier to the second input of the matrix multiplier with a delay of one space- time block; the matrix multiplier multiplying each space- time coded block by an immediately preceding differentially encoded space-time block to produce a current differentially encoded space-time block.
• the invention also provides a method of decoding symbols received in respective symbol intervals in response to transmission from T antennas of differentially encoded space-time blocks produced by the method recited above, comprising the steps of: providing T received symbols of each encoded space-time block; and producing decoded symbols X. in accordance with: Y.
• Yi is a vector of T symbols of a current encoded space-time block i
• Y-i is a vector of T symbols of an immediately preceding encoded space-time block i-l
• i is an integer
• k is a scaling constant
• H x is the T by T orthogonal space-time block coding matrix.
• Fig. 1 illustrates parts of a known space-time block code (STBC) transmitter
• Fig. 2 illustrates parts of a corresponding known receiver
• FIG. 3 illustrates parts of a known STBC transmitter using mapping and differential encoding
• Fig. 4 illustrates parts of an STBC transmitter using differential' encoding in accordance with an embodiment of the invention.
• Fig. 5 illustrates parts of a corresponding receiver in accordance with an embodiment of the invention.
• Fig. 1 illustrates parts of a known space-time block code (STBC) transmitter
• Fig. 2 illustrates parts of a corresponding known receiver.
• STBC space-time block code
• the transmitter of Fig. 1 includes a serial-to- parallel (S-P) converter 10, an M-PSK (M-ary phase shift keying) modulator or mapping function 12, and a space-time block coder (STBC) 14 providing outputs, via transmitter functions such as up-converters and power amplifiers not shown but represented in Fig. 1 by dashed lines, to at least two antennas 16 and 18 which provide transmit diversity.
• S-P converter 10 is supplied with input bits of information to be communicated and produces output bits on two or more parallel lines to the M-PSK mapping function 12, which produces from the parallel bits sequential M-PSK symbols xi, x 2 , ....
• the symbols i, x 2 , ..., represented by complex numbers, are supplied to the STBC 14, which for simplicity is shown in Fig. 1 as having two outputs for the respective transmit antennas 16 and 18, but may instead have more than two outputs for a corresponding larger number of transmit antennas.
• the STBC 14 forms a space-time block of symbols, as represented in Fig. 1, from each successive pair of symbols xi and x 2 supplied to its input.
• the STBC function is represented by a T-by-T orthogonal matrix H x , where T is the number of transmit antennas and hence symbol outputs of the STBC 14.
• T is the number of transmit antennas and hence symbol outputs of the STBC 14.
• the matrix H x can be more generally expressed as:
• the space-time blocks transmitted from the antennas 16 and 18 are received by an antenna 20 of the receiver shown in Fig. 2, producing received symbols yi, y 2 , ..., again represented by complex numbers, on a receive path 22. Pairs of these received symbols, y ⁇ ,i and ' y 2 ,i, alternatively represented as Yi, are supplied to a maximum likelihood decoder 24, shown within a dashed line box in Fig. 2.
• the decoder 24 comprises an STBC decoder 26 and an M-PSK demodulator 28.
• the STBC decoder 26 is supplied with the paired symbols Yi and also with channel estimates oci and ⁇ 2 , and produces estimates i_ ⁇ , x 2 , " ... of the transmitted PSK symbols x l r x 2 , ... respectively (the caret symbol denoting an estimate) . These estimates are supplied to the M-PSK demodulator 28, which produces estimates of the original input bits.
• the channel estimates oci and ⁇ 2 represent channel parameters or gains (amplitude and phase) of the channels from the transmit antennas 16 and 18, respectively, to the receive antenna 20, and are reasonably assumed to be constant over the duration of each space-time block.
• the channel estimates can be produced in any desired known manner, for example using pilot symbols also communicated from the transmitter to the receiver via the same channels .
• A is a vector of the channel estimates for the respective space-time block i then, excluding noise and interference, it can be seen that:
• the Alamouti publication extends this transmit diversity arrangement also to the case of more than one receive antenna, and this arrangement has also been extended for the case of more than two transmit antennas.
• Such known arrangements provide advantages of simplicity and diversity, but have the disadvantage of requiring channel estimation.
• Fig. 3 illustrates parts of an STBC transmitter as proposed by Tarokh et al . in the publication referred to above entitled "A Differential Detection Scheme for Transmit Diversity". This scheme avoids the disadvantage of requiring channel estimation as in the arrangement of
• the transmitter comprises a mapping function 30, a differential symbol calculation block 32, a delay 34, and two transmit antennas represented by a block 36.
• b l, 2, ..., signal point constellation
• This mapping, differential computation, and space-time block code transmission is repeated for subsequent blocks each of 2b bits, with the first two symbols of a transmission sequence providing a differential encoding reference and not conveying any information.
• Fig. 4 illustrates parts of a two-antenna STBC transmitter using differential encoding in accordance with an embodiment of this invention.
• this includes an S-P converter 10 to which input bits are supplied, whose output bits are supplied to an M-PSK mapping function 12 which produces sequential M-PSK symbols i, x 2 , ... represented by complex numbers.
• pairs x ⁇ ,i and x 2 ,i, or Xi, of these symbols are supplied to an STBC function 40, which forms the 2 by 2 orthogonal STBC matrix H x (Xi) as described above, in this case- scaled by a predetermined normalizing factor k as described further below.
• the output of the STBC function 40 is supplied to one input of a matrix multiplier 42, an output of which constitutes an STBC matrix H z ,i as described below and is supplied to the two transmit antennas 16 and 18 to be transmitted in a similar manner to that described with reference to Fig. 1 for the matrix H x (X ⁇ ).
• the matrix H z ,i is also supplied to an input of a delay unit 44, an output matrix H Z i_ ⁇ of which is supplied to another input of the matrix multiplier 42.
• each space-time block H Z/1 transmitted by the antennas 16 and 18 is equal to the normalized matrix kH x (X x ) produced by the function 40 multiplied in the matrix multiplier 42 by the matrix H z , ! -- ! of the previously transmitted space-time block, the latter being fed back to the multiplier 42 via the delay 44 (which provides a delay corresponding to one space-time block, i.e. two symbols in this case) .
• the matrix H Z/1 has the same properties as the matrix H 21 _ ! and these successive matrices can each be transmitted as a space-time block as described above.
• this equation (2) has a similar form to that of equation (1) above, except that the channel parameter vector Ai of equation (1) is replaced in equation (2) by kYi_ ⁇ . With this replacement, an arrangement for detecting the transmitted information can correspond to that described above with reference to Fig. 2.
• the decoding process the task of which is to solve the above equation (2), is in this case given by:
• the received symbol pair Yi is dependent only upon the normalization factor k which is predetermined and constant, the current space-time block code matrix H x (Xi), and the immediately preceding received symbol pair Yi-i ⁇
• the decoding of the received symbol pair Yi to produce the estimated decoded symbol X. is not dependent upon the channel parameter vector Ai, which is therefore not required to be estimated in order for the receiver to recover the transmitted information.
• the estimated decoded symbol X depends on the current received symbol pair Yi and the immediately preceding received symbol pair Yi-i, and error propagation in the decoded information is avoided because the decoding of each received symbol pair Yi is not dependent upon previously decoded information.
• Fig. 5 illustrates parts of a corresponding receiver in which, as in the known receiver of Fig. 2, the space-time blocks transmitted from the antennas 16 and 18 of the transmitter of Fig. 4 are received by the antenna 20 to produce received symbols yi, y 2 , ... on the receive path 22. From pairs of these received symbols, ⁇ ,i and
• the converted vector Y ⁇ is produced by a unit 50 and, via a delay unit 52, the matrix
• the decoder 56 does not use the channel parameter vector Ai, so that it does not require and is not dependent upon channel estimation, and that the decoder produces the estimates of the transmitted PSK symbols from two consecutively received signal blocks, so that there is no error propagation.
• the transmitter of Fig. 4 and the receiver of Fig. 5 are described above in the context of the transmitter having two antennas and the receiver having one antenna, it can be appreciated that embodiments of the invention are not limited to this case.
• the receiver may instead have two or more antennas signals from which are combined in a desired and appropriate manner, for example using maximal ratio combining.
• the transmitter may have more than two antennas, the STBC matrix H x (Xi) still being a T by T orthogonal matrix where T is the number of transmit antennas.
• a lower coding rate can be used.
• the 4 by 4 orthogonal matrix is derived from only 3 sequential M-PSK symbols xi, x 2 , and x 3 .
• the STBC matrix H x (Xi) can then be, for example, the matrix: which is orthogonal, i.e.:
• H x (x 1 ) ⁇ x (x 1 ) (
• I is the identity matrix.
• the normalization factor k for this matrix is 1/73.
• the transmitter for this example can be the same as described above with reference to Fig. 4.
• the task of the decoder is again to solve the equation: corresponding to equation (2) above, where in this case each of the vectors Y and Yi_ has four elements and the matrix H x (Xi) is a four by four matrix, so that this equation represents a set of four linear simultaneous equations.
• the receiver can have a generally similar form to that described above with reference to Fig. 5, except that the units 50 and 54 and the multiplier 58 are replaced by units for providing an explicit solution to this decoder equation.
• the size of the set of linear simultaneous equations represented by this decoder equation corresponds to the number T of transmit antennas and the corresponding size of the space- time block, and that an explicit solution to this equation can always be found regardless of the number T of transmit antennas .
• the performance of a system incorporating an arrangement in accordance with the invention can be further improved by concatenating differential STBC coding described above with a channel encoder, which may for example comprise a turbo coder of known form.
• a channel encoder which may for example comprise a turbo coder of known form.
• the input bits supplied serially to the S- P converter 10 or in parallel to the input of the M-PSK mapping function 12 may be derived, for example via a block interleaver of known form, from the output of a turbo coder also of known form.
• the estimated bits output from the decoder 56 can comprise soft values (probabilities or probability ratios) which are supplied, for example via a block de- interleaver of known form, to a channel decoder also of known form.
• Concatenation of turbo and STBC coding is known for example from G. Bauch, "Concatenation of Space- Time Block Codes and "Turbo"-TCM", Proceedings of the International Conference on Communications, ICC 99, pages 120

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• 2000
  • 2000-12-20 DE DE60035439T patent/DE60035439T2/de not_active Expired - Lifetime
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